The biological system includes a wide diversity of chemical species. The biological system sustains its activity by the systematic and ordered interactions or chemical reactions of these chemical species. The order is maintained by the molecular recognition functions of the molecules in the system. Such molecular recognition functions are considered to be the minimum required functions of the biologically functional molecules. In the body, the molecular recognition functions combine together in a highly sophisticated manner to exhibit various specific functions. There have been attempts to mimic these superior functions of the biological system for actual application. One example is the creation of artificial receptors, which have been actively researched and developed as novel functional material.
A molecular imprinting method is a technique that has caught attention these days as a method of synthesis of artificial receptors that can specifically recognize target molecules (see Non-Patent Publication 1). The molecular imprinting method is the technique whereby the shape of a molecule to be recognized (target molecule) is imprinted (impressed) in the polymer, and the resulting cavity (target-molecule recognition field) is used for molecular recognition. First, the target molecule is complexed with a molecule (functional monomer) that includes (i) a functional group(s) that specifically interacts with the target molecule, and (ii) a functional group, such as a vinyl group, that can polymerize with a cross linker. This is followed by radical polymerization with a cross linker. The target molecule is then removed from the polymer, and as a result binding sites complementary to the target molecule are constructed in the polymer. The resulting polymer is called a molecularly imprinted polymer.
The success or failure of obtaining a desirable molecular recognition function depends on how precisely the selective cavity (target-molecule recognition field) is constructed in the polymer. This is greatly influenced by the design of interaction patterns between the target molecule and the functional monomer. The patterns of interactions can be broadly classified into a non-covalent bonding type and a covalent bonding type. In the former, a target molecule and a functional monomer are complexed in a pre-polymerization mixture by non-covalent bonding such as hydrogen bonding or electrostatic interactions. In the latter, the complex is synthesized and isolated prior to polymerization with a cross-linker. These methods are selected according to the chemical properties of the target molecule, so that the optimum effect can be obtained. In the molecular imprinting method, the construction of the binding site by the functional monomer and the crosslinking monomer proceeds from the target molecule. This optimizes the binding site in terms of entropy, and enables the molecular recognition field to be tailored.
The molecular recognition polymer is highly useful as a sensing element when it has a sensing molecule that exhibits a signaling capability upon binding of its binding site (functional monomer) to the target molecule. For example, the inventor of the present invention has developed a molecularly imprinted polymer capable of sensing quenching of fluorescence that results from target binding, wherein a porphyrin-zinc complex was used as the functional monomer, and cinchonidine, which can be axially positioned to the functional monomer was used as the target molecule (see Non-Patent Publication 2).